Flying Gull

Gull plumages are a frequent source of despair to me.
I believe this is a ring-billed gull moulting out of 2nd winter basic plumage into adult alternate—there are the new inner primaries, and the new outer retrices without the terminal black band, to suggest moulting and which moult—but I certainly wouldn't bet detectable quantities of money on this point.

(It's a ring-billed because the white in the outer primaries is small in area; because the feet and legs are yellow; because the bill is also yellow with a black band near the tip, the bill is proportionately small but not that small, and because, aside from the black primaries, it's entirely white beneath. Also because I took the picture in Toronto, where, to a first approximation, all the gulls are ring-bills.)